The present invention relates to heating devices and controllers therefore, and in particular to a resistance heating device and its controller for heating a rod or post, such as a stud or threaded bolt.
The present invention has a particular application in the heating of studs or bolts, such as the bolts of a steam turbine casing. Although the invention is not limited to such an application, the invention will be described in such an environment and use.
It is desirable to heat the bolts used in a steam turbine casing during the fastening and unfastening thereof in that the bolt will elongate due to expansion during heating, allowing the nut to be threaded onto the bolt to a greater degree during the fastening process. When the bolt cools and shrinks, the nut is pulled tighter against the surrounding surface, thus assuring a secure fasting of the nut on the bolt and a clamping of the parts held together by the nut and bolt. In order to ease the removal of the nut from the bolt, such as during the servicing of the turbine, it is helpful to again heat the bolt to elongate it, in order to move the nut away from the surrounding surface, or at least to lessen the force holding the nut against that surface.
It has long been known to heat bolts, for example see U.S. Pat. No. 2,176,601, and to use induction heating to heat such bolts, for example, see U.S. Pat. Nos. 3,771,209 and 5,397,876.
Commonly available resistance type bolt heaters operate at 240 volts, and are individually controlled (on-off). These bolt heaters generally provide a heating power of about 50 watts per square inch. With the bolt heaters powered and controlled individually and with the given heating power output, the heating of a bolt used in a typical turbine requires generally 5 to 10 minutes or more of heating to elongate the bolt sufficiently to provide the necessary tightening or loosening of the bolt.
It would be an advance in the art if there were provided an easy to use resistance heating device and a method for heating multiple items, such as bolts.